Machine members which rotate, particulary at high speeds, need to be rotationally balanced. Otherwise, the imbalances cause forces to act on the member in a direction perpendicular to the axis of rotation resulting in undesired vibrations and uneven and accelerated wear of bearings which support such members. The usual solution is to statically balance the member by adding weights about the member, as is done to balance auto tires, or by removing weight about the member as by drilling holes at the appropriate locations. As long as the magnitude and direction of the forces and loads applied to the rotating member are relatively constant, static balancing usually results in smooth rotation of the member.
When forces are applied to a rotating member in a pulsed manner or in such a manner that the direction of such forces changes during application of the forces, as happens with the crankshaft of an internal combustion engine and in other machinery, such forces have the potential for inducing rotational imbalances in the rotating member even if the member is statically balanced. Crankshaft counterweights, a flywheel, and often a harmonic balancer are affixed to a crankshaft to minimize the vibrations which result from the nature of the forces which act on the crankshaft. In a reciprocating piston engine, the power pulses or firing strokes are pulsed and cyclic. However, the effects of the counterweights, flywheel, and harmonic balancer are continuous. In engines with relatively numerous cylinders, such as six, eight, or more, the power pulses are more evenly spaced over the crankshaft rotation cycle. In engines with relatively fewer cylinders, such as four, two, or one, it is difficult to evenly time the power pulses whereby these engines tend to vibrate more than engines with more cylinders.
Dynamic rotational balance devices have been developed to overcome some of the limitations of static balancing of rotational members. In general, a dynamic balancer consists of a circumferential race or groove in which are placed movable weights, usually spherical and metal. As the member on which the dynamic balancer is affixed rotates, the weights shift circumferentially to counteract rotational imbalances that occur. Many of the rotational balance devices that have been proposed while theoretically capable of performing their desired function, are configured such that the manufacture of such devices would be difficult and expensive. For example, several known balance devices of this type employ races formed of circular tubes. It would be difficult to shape such a tubular member as a precise circle. Failure to precisely shape such a race member could contribute to the rotational imbalance problem. At least one known balance arrangement for railway wheels provides an annular cavity which is formed when the wheel is cast. It is proposed that the cavity could be formed by a mold core of sand which is later removed. While such a manufacturing process is possible, it would be expensive and could not be carried out with a great degree of precision or smoothness of the race.